1. Field of the Invention
The present invention relates to a reflective liquid crystal display device and a substrate for the reflective liquid crystal display device used for the device, and more particularly, to the reflective liquid crystal display device of VA system providing a diffusion reflective plate having uneven shape and using liquid crystal of negative dielectric anisotropy, and a substrate for the reflective liquid crystal display device used for the device.
2. Description of the Related Art
The reflective liquid crystal display device has characteristics of thin shape, light weight, and low power consumption, and does not need a back light unit because of the display using external light so that it is possible to realize a thin paper-like display device. The reflective liquid crystal display device put to practical use nowadays adopts a one sheet polarizing plate system disclosed in Japanese Patent Laid-Open No. 232465/1993 and Japanese Patent Laid-Open No. 338993/1996. Since high contrast is obtained and only one sheet of polarizing plate is used by the one sheet polarizing plate system, loss of light is reduced and a comparatively bright display is obtained.
FIG. 21 shows an outline of the reflective liquid crystal display device 100 construction according to a one sheet polarizing plate system of the related art, and shows a section cut off vertically to a display surface. A diffusion reflective plate 102 having uneven reflective surface is formed on a substrate not shown. A phase difference plate (λ/4 plate for example) 106 is arranged on a facing substrate not shown through the reflective surface of the diffusion reflective plate 102 and the predetermined cell gap. A polarizing plate 108 is arranged at an external light incidence side of the phase difference plate 106. A liquid crystal layer 104 is sealed in the predetermined cell gap, and horizontal alignment films (not shown) are formed at boundaries of the liquid crystal layer 104 of both substrates. A rubbing treatment, for example, is performed on the two horizontal alignment films sandwiching the liquid crystal layer 104 so that liquid crystal molecules of the boundaries are orientated in different directions to each other.
For the liquid crystal layer 104, nematic liquid crystal having positive dielectric anisotropy is used. Since liquid crystal molecules of the horizontal alignment film boundary are oriented almost horizontally at the predetermined pre-tilt angle in the condition that voltage is not applied between both substrates, the liquid crystal becomes twist nematic liquid crystal in which liquid crystal molecules are rotated toward a cell gap direction. The conventional reflective liquid crystal display device 100 of one sheet polarizing plate system performs display of normally white type displaying bright condition (that is, white) when no voltage is applied.
FIG. 22A and FIG. 22B show conditions of liquid crystal molecules 104a in the liquid crystal layer 104 of the conventional reflective liquid crystal display device 100 of one sheet polarizing plate system. FIG. 22A shows the liquid crystal molecules 104a in the bright condition when no voltage is applied, and FIG. 22B shows the liquid crystal molecules 104a in the dark condition when voltage is applied. For easy description, condition of the crystal molecules 104a without twist is drawn simplifying the figure in FIG. 22A and FIG. 22B. In the dark condition, the liquid crystal molecules 104a of the horizontal alignment film boundary does not rotate much to the vertical direction from horizontal direction even if voltage is applied because of anchoring effect (alignment regulation) as shown in FIG. 22B. Because of that, retardation (phase delay) occurs at this area, and a problem of not enough black display. Then, a method is proposed, in which a phase difference plate is designed considering residual retardation at voltage applying condition (Y. Itoh, N. Kimura, S. Mizushima, Y. Ishii, and M. Hijikigawa, AM-LCD 2000 digest, p. 243 (2000)). However even if such design is performed, it is difficult to remove the retardation at all wavelengths and to obtain high contrast.
FIG. 23 shows an outline of the reflective liquid crystal display device 101 construction according to one sheet polarizing plate system of the related art differing from the device of FIG. 21, and shows a section vertically cut off to a display surface. A point differing from the structure shown in FIG. 21 is that vertical alignment films (not shown) are formed at boundaries of the liquid crystal layer 105 of both substrates and nematic liquid crystal having negative dielectric anisotropy is used for the liquid crystal layer 105. Thereby, a liquid crystal display device of VA (vertically aligned) type in which liquid crystal molecules of vertical alignment film boundaries orient almost vertically at the condition when no voltage is applied between both substrates is constructed. The conventional reflective liquid crystal display device 101 of one sheet polarizing plate system performs display of normally black type displaying, dark condition (that is, black) when no voltage is applied. The conventional reflective liquid crystal display device 101 can obtain very high contrast because residual retardation does not exist at a black display. However, since retardation generates for incident light from an oblique direction to a display surface, a problem that it is difficult to obtain a good dark condition in external incident light to the display surface from all directions occurs. Especially, since light reaching the diffusion reflective plate 102 is further diffused and reflected there by using the diffusion reflective plate 102 having an unevenness, influence of light by an oblique incidence becomes large so that contrast falls remarkably.